1. Field of the Invention
This invention relates to a method and apparatus for recording and/or reproducing information signals for an optical disc. More particularly, it relates to an optical disc recording and/or reproducing method and apparatus for recording or reproducing information signals for an optical disc as a servo control operation s performed.
2. Description of the Related Art
Up to now, a disc-shaped recording medium for recording information signals, such as optical disc, is in widespread use. The optical disc recording and/or reproducing apparatus is configured for illuminating laser light on an optical disc from an optical pickup to record or reproduce information signals on or from the signal recording surface of the recording medium.
Recently, there is an increasing demand for a high recording density optical disc having a large data storage capacity. This high density recording is possible by reducing the diameter of a light spot of a laser light beam illuminated from an optical pickup on the signal recording surface.
The light spot diameter is changed by e.g., the wavelength of the laser light beam and by the numerical aperture NA of an objective lens. It is noted that the diameter of the light spot focussed on the signal recording surface is proportionate to the wavelength of the laser light and inversely proportionate to the numerical aperture NA of the objective lens.
Thus, high density recording on an optical disc becomes possible by shortening the wavelength of the laser light or by increasing the numerical aperture NA of the objective lens.
On the other hand, the depth of focus of the light spot is proportionate to its wavelength and inversely proportionate to the square of the numerical aperture NA of the objective lens. Since the depth of focus is reduced in case of using means for enabling high recording density, it is necessary to reduce the focussing error for assuring recording/reproduction.
Moreover, since the track pitch is narrowed by increasing the recording density, it is also necessary to reduce the tracking error for assuring recording/reproduction.
Thus, for realization of high recording density, suitable measures need to be used to reduce the follow-up errors of focussing servo and tracking servo that are mainly responsible for occurrences of focussing and tracking errors.
It is noted that, if a shock is applied from outside to a device, there are produced transient errors which need to be suppressed. Although it is possible to increase the servo followup performance by broadening the servo band and increasing the gain, steady-state servo stability is impaired by increasing the gain thus producing negative effects.
Specifically, should the servo band and the gain be increased to improve servo followup characteristics, there is raised a problem that the servo followup error is increased against expectations, such as by excess reaction to noise components of a higher frequency band.